1. Field of the Invention
The present invention relates to a technology concerning a light emitting device having a light emitting element.
2. Description of Related Art
The introduction of digital technology into equipment and systems used in broadcasting has been progressing. Research and development is being performed in many countries toward achieving the digitalization of broadcast waves, that is, achieving digital broadcasting.
Further, in response to the digitization of broadcast waves, research and development of active matrix display devices capable of displaying an image using a digital video signal having image information (digital video signal) as it is, without converting the digital signal into an analog signal, has also been extensively progressing in recent years.
There are typically two driving methods available for performing gray scale display by using binary voltage signal of the digital video signal, including an area ratio gray scale scheme and a time gray scale method.
The area ratio gray scale scheme is a driving method that performs gray scale display by dividing one pixel into a plurality of sub-pixels, and independently driving each of the sub-pixels based on a digital video signal. One pixel must be divided into a plurality of sub-pixels with this area ratio gray scale scheme. In addition, in order that the divided sub-pixels are driven independently, it is necessary to form pixel electrodes corresponding to each of the sub-pixels. A problem develops in that the pixel structure becomes complex.
On the other hand, the time gray scale method is a driving method that performs gray scale display by controlling the length of turn-on of the pixels. Specifically, one frame period is divided into a plurality of display periods arranged on a time axis in a predetermined order (display periods). Each of the display periods is weighted to correspond with a gray scale level, and by turning on or turning off each pixel according to the digital video signal, gray scale display is performed. That is, the gray scale of a pixel can be found by integrating the lengths of the display periods within the one frame period during which the pixel turns on, from among all of the display periods that appear within the one frame period.
The development of light emitting devices using light emitting elements has been progressing in recent years. In addition to the advantages of currently existing liquid crystal display devices, the light emitting devices also have characteristics such as high response speed, superior dynamic display, and a wide field of view, and attract attention as next generation small size mobile flat panel displays capable of utilizing dynamic contents.
A wide variety of materials such as organic materials, inorganic materials, thin film materials, bulk materials, and dispersion materials are used for the light emitting elements. Among those, organic light emitting diodes (OLEDs) that are formed mainly of organic materials can be given as typical light emitting elements. The OLEDs have a structure with an anode, a cathode, and a light emitting layer that is sandwiched between the anode and the cathode. The light emitting layer is formed of one or a plurality of materials selected from the aforementioned materials. Further, luminescence in the light emitting layer includes light emission when returning to a base state from a singlet excitation state (fluorescence) and light emission when returning to a base state from a triplet excitation state (phosphorescence). The OLED response speed is generally high as compared with that of liquid crystals and the like, and therefore the time gray scale method is applied.